Finding gaps in sequence of numbers

I have a std::vector containing a handful of numbers, which are not in any particular order, and may or may not have gaps between the numbers - for example, I may have { 1,2,3, 6 } or { 2,8,4,6 } or { 1, 9, 5, 2 }, etc.

I'd like a simple way to look at this vector and say 'give me the lowest number >= 1 which does not appear in the vector'. So,

for the three examples above, the answers would be 4, 1 and 3 respectively.

It's not performance critical, and the list is short so there aren't any issues about copying the list and sorting it, for example.

I am not really stuck for a way to do this, but my STL skills are seriously atrophied and I can feel that I'm about to do something inelegant - I would be interested to see what other people came up with.